The present invention relates to a field instrument. More specifically, the present invention relates to an input circuit in the field instrument which receives power from a communication loop.
Process control systems are used in manufacturing and other process control environments to monitor and control the operation of processes. In a conventional process control system, instruments are placed in the field to implement the process control. Such instruments (which are typically referred to as field instruments or transmitters) may typically include a peripheral device, and circuitry to support the peripheral device. The field instrument is coupled to a communication loop (i.e., a network segment of the process control system) and transmits information over the communication loop to other portions of the process control system. The communication loop is typically a two-wire loop which provides power for operation of the field instrument. Communication is accomplished over the loop through a field bus standard which is a digital communications protocol.
The field instruments may typically include a transmitter, a process variable control circuit or a field mounted controller. A transmitter includes a sensor which is placed in the field and monitors a variable of the process, such as pressure, temperature or flow. The transmitter is coupled to the communication loop and transmits information over the communication loop to a controller which monitors operation of the process. Transmitter communication using the field bus standard allows more than one transmitter to be coupled to a single communication loop to transmit the sensed process variable to the control room. The field bus standard describing this communication is described in ISA 50.02-1992 Section 11. The HART.RTM. standard is another communication standard which allows digital communication over a 4-20 mA process variable signal.
If the field instrument includes a process variable control circuit, the peripheral is typically a valve or other controllable device for controlling some aspect of the process. The position of the valve, along with other desired parameters, are monitored and the valve is controlled by the process variable control circuit to a desired position. Conventional process variable control circuits include a microprocessor, a microcontroller, or other suitable controller which performs calculations and signal correction, and which also facilitates communication over the communication If the field instrument is a field mounted controller, it does not include a peripheral device per se. Instead, it is coupled to the communication loop and may typically poll a plurality of transmitters to receive a signal indicative of a sensed parameter or parameters. The field mounted controller then performs a desired calculation and provides a resultant signal over the communication loop for use in other portions of the process control system.
In any case, the field instrument is powered by the communication loop either by an AC current, or by a DC voltage. When an AC current is applied, an input circuit in the field instrument rectifies the AC signal so that it can be provided to the remainder of the circuitry in the field instrument. When a DC voltage is applied, it is desirable that the field instrument be configured so that the DC voltage supply can be connected to the field instrument in either polarity (i.e., the field instrument will operate regardless of the particular orientation taken by the positive and negative power supply terminals relative to the field instrument).
Prior field instruments have included a diode bridge rectifier circuit. However, such a circuit introduces a voltage drop thereacross of approximately 0.6 volts to 1.4 volts. Therefore, the voltage available to the rest of the field instrument is significantly less than that provided by the terminals of the power supply. This reduces both the power efficiency and the maximum output voltage of the bridge rectifier.